The "Kaminsky catalyst" is well known as a homogeneous catalyst system for olefin polymerization. This catalyst has an extremely high polymerization activity, and provides a polymer having a narrow molecular weight distribution.
Of the transition metal compounds used for the Kaminsky catalyst, for example, ethylenebis(indenyl)zirconium dichloride and ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride has been known to be used for producing isotactic polyolefins, as described in Japanese Patent Laid-Open Publication No. 130314/1986. However, polyolefins prepared by the use of such catalyst generally have a low stereoregularity and a low molecular weight. If low temperatures are used, polymers having a high stereoregularity or a high molecular weight can be obtained, but a problem of low polymerization activity is involved.
It has been known that use of hafnium compounds in place of the zirconium compounds makes it possible to prepare high molecular polymers, as described in Journal of Molecular Catalysis, 56 (1989), pp. 237-247, but this process involves a problem of low polymerization activity. Further, compounds such as dimethylsilylbis-substituted cyclopentadienylzirconium dichloride have also been known from Japanese Patent Laid-Open Publication No. 130704/1989 and Polymer Preprints, Japan, vol. 39, No. 6, pp. 1,614-1616 (1990), but there is a problem in that they do not satisfy all of the high polymerization activity, and the high melting point and molecular weight at the same time.
Japanese Patent Laid-Open Publication No. 268307/1992 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula ##STR2## and an aluminoxane.
Further, EP 0 530 648 A1 describes another olefin polymerization catalyst formed from a metallocene compound represented by the following formula ##STR3## wherein A is a lower alkyl group, and an aluminoxane.
However, the polyolefins obtained by the use of these catalysts are still insufficient in the melting point, the molecular weight, etc. In this connection, a catalyst containing a compound of the above formula wherein A is a phenyl group is disclosed by Hoechst AG. in 40 YEARS ZIEGLER CATALYSIS IN HONOR OF KARL ZIEGLER AND WORKSHOP (Sep. 1-3, 1993). However, there remains a problem that the polyolefins obtained by the use of this catalyst still have a low melting point and a low molecular weight.
Under these circumstances, the advent of an olefin polymerization catalyst and a process for olefin polymerization, each of which shows a high olefin polymerization activity and produces polyolefins of excellent properties, is desired. Further, also desired is the advent of a catalyst component which can be used for such olefin polymerization catalyst and a novel transition metal compound which can be used as the olefin polymerization catalyst component.